1. Field of the Invention
The present invention relates to a fast response membrane generator using heat accumulation.
2. Description of the Background
Membrane processes are presently used in a wide variety of gas separators. In such processes, the feed stream is brought into contact with the surface of the membrane with the more readily permeable component being recovered at low pressure, whereas the less readily permeable component is collected as a non-permeate stream at a pressure close to the feed pressure.
Membrane systems are increasingly employed, for example, for on-site production of nitrogen or other gases. In all such cases, the membrane generator is used in combination with a liquid nitrogen back-up tank that is used at start-up until the production reaches the required purity as requested by the customer, during peak demands or generator shutdown periods. Unfortunately, the efficiency of such membrane systems is reduced by variable customer demand over time. In particular, when the required flow rate decreases, the system is utilized only part-time so that stops and restarts are frequent. Furthermore, long-term shutdowns frequently occur due to required regular maintenance. Thus, liquid nitrogen must be vaporized at each restart to feed the customer line before the generator has reached the required level of purity. This vaporized liquid nitrogen is, in any case, more expensive than the nitrogen produced by the generator. Thus, there is a great need to reduce the consumption thereof to a minimum. It is also especially desired to reduce the start-up time to a minimum.
One of the limiting factors for reaching the required purity is the time required for the membranes to reach operating temperature when this temperature is above ambient temperature. In cases, where the membrane operating temperature is above ambient temperature, it is, therefore, necessary to be able to introduce a large quantity of heat into the system in a very short period of time, such as a few seconds. Unfortunately, at present, several minutes are required to reach operating temperature. Most of the time required to bring the system to operating temperature is due to the inertia of the heater, heating of the pipes and membranes that are cold.
At present, in order to reduce start-up time, a system has been proposed which uses an electrical gas heater on the feed pipe. Although very powerful heaters may be used, this amounts to over-design at excessive cost when compared to the power required merely to keep the system at operating temperature.
Another solution proposed is to maintain the entire system at the required temperature by maintaining a high ambient temperature. However, this turns out to be quite energetically expensive, particularly for long-term shutdown periods. Even if only the sensitive part, i.e., the membranes, are enclosed in a heated insulated box as in U.S. Pat. No. 4,787,919, the cost may still be high for long-term shutdown periods. Moreover, this does not improve the time necessary to heat the pipes and other equipment that is installed between the heater and the membranes. Further, if the air feed to the system is cold, the membranes will not work at optimum even if they are already at the operating temperature. Additionally, since the mass of the membrane is small compared to the feed flow rate, the system operates with the inefficiency of a cold system if the feed air is cold.
Thus, a need exists for a membrane generator system that can be restarted in a short amount of time, reaching operating temperature quite rapidly without any additional cost in energy or in investment.